Fluoroelastomers, especially perfluorinated elastomers, are used in a wide variety of applications in which severe environments are encountered, specifically end uses where exposure to high temperatures and aggressive chemicals occur. For example, these polymers are often used as seals for aircraft engines, in semiconductor manufacturing equipment, in oil-well drilling devices, and in sealing elements for industrial equipment used at high temperatures.
Inorganic particles have been added to fluoropolymer compositions as fillers and/or to improve the final properties of the fluoropolymer article.
There are many papers which disclose adding inorganic particles and even inorganic nanoparticles as fillers to fluoropolymer dispersions prior to coagulation. One advantage is that a more uniform blend of the filler can be achieved. For example, Malvasi et al. (U.S. Pat. No. 7,691,936) discloses adding organic or inorganic fillers into a polytetrafluoroethylene or modified polytetrafluoroethylene dispersion and then coagulating. This process is said to produce good homogeneity and optimal distribution of the fillers in fluoropolymer fine powders. Malvasi et al. discloses coagulation occurring with the usual known methods of the prior art for the fluoropolymer dispersion, without the need of plant modification.
The traditional methods of coagulating fluoropolymer dispersions include: physical and chemical methods. In physical methods the dispersion may be subject to strong (high) shearing using a stirring device thereby coagulating the particles, (typically by rotor stator having shear rates in excess of 1000 (l/s)). Another method of physical coagulation is the freeze-thaw method. The dispersion is cooled sufficiently to freeze it, which destabilizes the dispersion so that on thawing, the coagulate separates from the liquid. Generally, this technique is not preferred for scale-up due to the scaleability and intensive energy requirements. In chemical coagulation, an electrolyte or inorganic salt is added to the dispersion so that the stability of the dispersion is decreased thereby causing coagulation.
Among these methods, it is preferable to use the chemical coagulation method wherein an electrolyte or inorganic salt is added to the polymer dispersion. Examples of electrolytes used to chemically coagulate fluoropolymer primary particles include HCl, H2SO4, HNO3, H3PO4, Na2SO4, MgCl2, Al2(SO4)3, and ammonium carbonate. Among these compounds, it is preferable to use compounds which can volatize during the process of drying the coagulate. Examples of inorganic salts used to chemically coagulate fluoropolymer primary particles include alkali metal salts, alkaline earth metal salts, and ammonium salts, of nitric acid, hydrohalic acid, phosphoric acid, sulfuric acid, molybdate, monobasic or dibasic sodium phosphate, ammonium bromide, potassium chloride, calcium chloride, copper chloride and calcium nitrate. These electrolytes and inorganic salts may be used independently or in combinations of two or more.